Speaker
Description
There is growing interest in laser-based, all-optical technologies for particle acceleration, due to their intrinsically compact nature, and to the unique properties of the beams produced through these techniques. The prospect for future clinical application has been a strong motivation for research in acceleration of protons and heavier ions. In particular, the ultrashort nature of the ion pulses makes their application in in-vitro and in-vivo radiobiology particularly interesting, in the context of the current interest in FLASH irradiations at very high dose rates. While the most established acceleration mechanism is via the so-called Target Normal Sheath Acceleration (TNSA), several alternative mechanisms have attracted attention, either due to the capability of acting on bulk ions within the irradiated targets, or to an enhanced acceleration efficiency and faster scaling with increasing laser intensity.
Particularly promising are interactions with ultrathin targets where, depending on the irradiation conditions, the acceleration can be dominated by radiation pressure, or by relativistic transparency effects. A series of experimental campaigns carried out by our group and collaborators on the GEMINI laser system at the Central Laser Facility (RAL) has investigated the acceleration of protons and carbon ions from ultrathin (nm-scale) carbon foils. This work has highlighted strong dependences of the ion energies on target thickness and laser polarization, with particularly noticeable effects on carbon ions. Perspectives for scaling up these results to the multi-PW regimes (e.g. accessible on the ELI and Apollon facilities) will be discussed. The role of target pre-expansion is emerging as particularly important for the acceleration process and simulations indicate the potential for significant acceleration enhancement by exploiting suitable density profiles.
We will also discuss a variant of TNSA acceleration which has recently been proposed, in which the electrons driving the process are accelerated by surface plasma waves excited on extended targets by a laser pulse propagating parallel to the target surface. Recent experiments by our group, also performed on GEMINI have characterized the electron source resulting from this configuration, revealing a strong highly collimated, superponderomotive component, which is an important first step for future extension to ion acceleration.